The use of a hot staking technique for joining pieces of material together is well known in the art. In the hot staking process, a first piece is formed with a plurality of spaced apertures along the edge to which a second piece of plastic is to be joined. The second piece has a plurality of spaced studs which project from the surface of the piece and in assembly are inserted in the apertures in the first piece. Each individual stud is then heated and shaped by a suitable metal tool to form a rivet-like end on the stud to lock the two pieces together. While this sounds like a relatively simple process to carry out, great care must be exercised in the heating of the stud in order to properly form a strong joint between the two parts. Too much heat tends to cause the stud to begin to flow and shaping the stud will not remove enough of the heat quickly enough to cause the stud to lock the parts together, resulting in a loose joint. If the stud is not heated enough before being shaped, the stud will tend to develop stress and the plastic base will show stress weakness as discoloration and multiple rings surrounding the stud.
The method of applying heat to the stud is also important. In the past, it has been common for the shaping tool to be heated and then applied to the stud where it transfers heat to the stud, softening the stud as the stud is being shaped. This technique results in stress in the stud as the stud is not heated properly before pressure is applied and also the heated staking tool tends to soften the plastic resulting in plastic being withdrawn from the shaped stud by the tool. When the shaping tool is withdrawn, thin hair-like strands of plastic can be drawn away from the stud which, in a proper operation, should be cleaned from the staking tool before the next staking operation is carried out or else a very poor quality product will result.
In another technique, heat is applied directly to the stud in the form of hot air and after the stud is warm, a first tool in the form of a clamp comes down camming the heat duct out of the way before a second shaping tool comes down to stake or shape the softened stud. The overall result of this technique is a very complex machine.
Through the experimentation leading to the development of the present invention, the inventors determined that the key to the hot staking process is the proper preparation of the stud before shaping and the proper application of pressure to the stud by a cool staking tool. If the stud is heated to just below the temperature where it will begin to undergo flow, and if this heating process is carried out very quickly, then the staking tool will be able to shape the part without inducing stress into the part and/or the underlying plastic material being joined. The inventors also found that pressure cannot be traded off for heat. If an attempt is made to speed up the process by shortening the heating time and increasing the pressure, a very poor joint is formed which is subject to stress fatigue and discoloration in the parts being joined together.